The present invention relates to a process for producing 2,5-diamino-6-nitropyridine derivatives, their use in hair dyes and novel 2,5-diamino-6-nitropyridine derivatives.
Nitro dyes are in widespread use in hair dyes today. They are used in oxidation dyes as additives for the production of natural or modern shades. However, by combining several differently coloured nitro dyes it is also possible to produce hair dyes that are capable of dyeing the hair in natural or modern shades without the application of oxidizing agents.
Thus, for example, brown colorations having a natural effect can be produced by combining a yellow-dyeing nitro dye with a red-dyeing nitro dye and a blue-dyeing one. However, in addition, it is also possible to obtain modern brown shades, for example, with a yellow-dyeing nitro dye and a violet one. Therefore, nitro dyes that cover practically the entire colour spectrum from yellow via orange, red and violet to blue are required. In order to attain homogeneous colorations at the hairline and at the hair ends and for hair damaged in various ways, it is advantageous when these dyes originate from a group of chemical compounds having the same parent substance since similar physical properties that determine the absorptive power are then more readily attainable.
In addition, hair dyes must also satisfy many other requirements.
In toxicological and dermatological respects the nitro hair dyes must be reliable. A prerequisite for their use in oxidation hair dyes is that they remain stable in the presence of hydrogen peroxide in alkaline solution. Furthermore, a good light fastness, acid resistance and rubbing fastness are required for the hair colorations produced. Finally, it should be possible or produce the nitro dyes on an industrial scale by processes as simple as possible.
Nitro dyes are described in greater detail in the literature (see, for example J. C. Johnson, Hair Dyes, Noyes Data Corp., Park Ridge, N.J., U.S.A. 1973, page 43 to 91 and 113 to 124). Heretofore, it has been possible to produce yellow, red, violet and blue dyes only by means of differently substituted 2-nitro-p-phenylene diamines. Since doubts have recently been cast upon the physiological reliability of some of the 2- nitro-p-phenylene diamine derivatives there exists a great need for dyes which have a non-benzoid parent substance and better physiological properties at a color band width as identical as possible.
Hair dyes having a content of nitropyridine derivatives are known from the literature. Thus, for example, diamino-pyridine derivatives are generally described in DE-PS 1949750. However, only 2,3-diamino-5-nitro-pyridine derivatives which dye human hair bright yellow to orange and thus have only a limited color spectrum are mentioned as concrete examples.
Furthermore, agents for the oxidative dyeing of hair are described in DE-OS 3,334,030. These agents are said to contain specific diamino-nitropyridine derivatives having a general formula. However, no specific diamino-nitro-pyridine derivatives are mentioned in this publication. Since in DE-OS 3,334,030 nitro-pyridines are fundamentally applied jointly with developing components and are thus intended to react as coupling components within the scope of an oxidative hair dye system, they are selected according to their coupling property.
Unlike these nitropyridines the 2,5-diamino-6-nitropyridines described in the present application can, of course, also be applied as oxidation hair dyes. However, they do not react within the system but are applied as additional direct dyes. Therefore, the principal range of application of these dyes is the use in direct dyes of the type of shades that do not require the addition of oxidizing agents. The present 2,5-diamino-6-nitro-pyridine derivatives thus are not all applicable within the scope of DE-OS 3,334,030 and also have a chemical structure that differs completely from that of the nitro coupling elements described in the Examples.
The concrete Examples described in DE-OS 3,334,030, which also result in reddish shades, are based on an oxidative hair coloration and are due substantially to a self-reaction of the 2,5-diamino-pyridine used in said Examples. Furthermore, the nitro compounds described in this German publication are not resistant to reduction in the presence of dithionite.
Thus, no non-benzoid system of direct nitro hair dyes that can dye the hair in shades from orange to blue violet is known from the literature.